update to latest react-router (2.0.0-rc4 as of writing)

[bencooling]

Update react-router
npm install react-router@beta

npm start throws an error
Invariant Violation: Element type is invalid: expected a string (for built-in components) or a class/function (for composite components) but got: undefined.

Also thank you for react-router!

[bencooling]

Submitted #5 to fix this issue.